1. Field of the Invention
The present invention relates to dual antenna mobile computers such as RF computer terminals connected to wireless networks, and more particularly to receivers using I and Q channel demodulators in such computers.
2. Background of the Invention
Wireless local area networks use infrared or radio frequency communications channels to communicate between portable or mobile computer terminals and stationary access points or base stations. These access points are in turn connected by a wired (or possibly wireless) communication channel to a network infrastructure which connects groups of access points together to form a local area network, including, optionally, one or more servers or host computer systems.
One increasingly important type of mobile computer are those which are coupled to or incorporates a bar code symbol reader, and are now in very common use for portable data collection applications. Typically, a bar code symbol comprises one or more rows of light and dark regions, typically in the form of rectangle. The relative widths of the dark regions, the bars and/or the widths of the light regions, the spaces, encode data or information in the symbol. A bar code symbol reader illuminates the symbol and senses light reflected from the regions of differing light reflectivity to detect the relative widths and spacings of the regions and derive the encoded information. Bar code reading data collection application software, executing on such mobile computers, improve the efficiency and accuracy of data input for a wide variety of applications. The ease of data input in such systems facilitates more frequent and detailed data input, for example, to provide efficient taking of inventories, tracking of work in progress, and make use of applications programs that may communicate to and interoperate with other applications programs operating on a remote host or server which the mobile computer communicates with through a wireless network.
Wireless and radio frequency (RF) protocols are known which support the logical interconnection of mobile computers and roaming terminals having a variety of types of communication capabilities to host computers. The logical interconnections are based upon an infrastructure in which at least some each of the remote terminals are capable of communicating with at least two of the access points when located within a predetermined range therefrom, each terminal unit being normally associated with and in communication with a single one of such access points. Based on the overall spatial layout, response time, and loading requirements of the network, different networking schemes and communication protocols have been designed so as to most efficiently regulate the communications between a given terminal and the network through the selected access point. One such protocol is set forth in the ISO/IEC 8802-11, or ANSI/IEEE Std 802.11 entitled “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications” (1999 edition) available from the IEEE Standards Department, Piscataway, N.J. (hereinafter the “IEEE 802.11 a Standard”).
The IEEE 802.11 a Standard also specifies the PHY layer operating in the 5 GHz band, which is open to unlicensed devices in the U.S. The IEEE 802.11 a Standard is based on orthogonal frequency division multiplexing (OFDM) to modulate the data. Digital data is divided among a large number of adjacent carriers so that a relatively small amount of data is carried on each carrier. Adjacent carriers are mathematically orthogonal. Their sidebands may overlap but signals can be received without adjacent carrier interference. The main benefit of OFDM modulation is it robustness to multipath echoes, which are encountered in the indoor and mobile environments. Each OFDM symbol is composed of fifty-two non-zero subcarriers of which forty-eight are data subcarriers and the remaining four are carrier pilot subcarriers. The PHY specifications encompass data rates from 6 Mbit/s up to 54 Mbit/s with 20 MHZ spacing between adjacent channels. All implementations are required to support 6, 12 and 24 Mbit/s. Optional extensions are for 9, 18, 36, 48 and 54 Mbit/s. The range of data rates is provided to match the wide range of radio channel characteristics in both indoor and outdoor environments. The multirate mechanism of the MAC protocol ensures that all devices communicate with each other at the best data rate in the present channel.
In a conventional single-carrier digital communication system, data symbols are transmitted serially using some modulation scheme, and the spectrum of each symbol is allowed to occupy the entire channel bandwidth. In multicarrier modulation schemes, data symbols are transmitted in parallel on multiple subcarriers that share the channel bandwidth using some form of frequency-division multiplexing (FDM). The modulation scheme on one subcarrier may be chosen independent of that used on other subcarriers. Thus, subcarriers in frequently segments of the channel with a high signal-to-noise ratio (SNR) may use high-rate modulation, while those with degraded SNR use low-rate modulation, or are not modulated. Systems which adaptively load the subcarriers differently depending on the spectral shaping of the channel are common in wired applications such as asymmetric digital subscriber lines (ADSL), and the technique is usually referred to as discrete multitone or DMT. DMT systems have been widely analyzed and reported in the prior art.
In OFDM the spectra of the subcarriers overlap, and their spacing is chosen so that each subcarrier is orthogonal to all other subcarriers. The common method of obtaining orthogonality of subcarriers is to choose their frequency spacing equal to the inverse of the subcarrier symbol duration. Baseband processing of the OFDM signal is then conveniently effected using the discrete Fourier transform, implemented as an inverse fast Fourier transform (IFET) and a fast Fourier transform (FFT) that modulate and demodulate parallel data blocks, respectively. The set of subcarriers generated during one transform defines an OFDM symbol. The subcarriers are conveyed by serial transmission over the channel of the time samples generated by the IFFT. The duration of the OFDM symbol, which is the same as that of the subcarrier symbol, is thus equal to the time window of the transform.
Prior to the present invention, there has not been a simple, automatic technique and circuit would allow mobile computer to select one out of two antennas to maximize reception of the signal.